The long-range purpose of this project is to study the effects of ionizing and ultra-violet radiation on nucleic acids and proteins and to investigate the mechanism of action or radioprotective and radiosensitizing chemicals, since these can be potentially useful in radiation therapy. The present report includes electron spin resonance studies of the radicals produced by gamma-irradiation of aqueous solutions of nucleic acids, proteins and their constituents. The effect of visible and ultra-violet light on gamma-irradiated wet DNA at 77 degrees K were investigated.